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Related Concept Videos

¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

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Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are...
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Proton (¹H) NMR: Chemical Shift01:07

Proton (¹H) NMR: Chemical Shift

4.1K
Organic molecules primarily contain carbon and hydrogen atoms. While all the hydrogen isotopes are NMR-active, protium or hydrogen-1 is the most abundant. It has a significant energy separation between its nuclear spin states due to its large gyromagnetic ratio. As per Boltzmann's distribution, an increase in the energy separation implies a greater excess population of nuclei available for excitation, resulting in a strong NMR absorption signal.
Absorption signals of all the protium nuclei...
4.1K
¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)01:20

¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)

1.8K
When proton-coupled carbon-13 spectra are simplified by a broadband proton decoupling technique, structural information about the coupled protons is lost. Distortionless enhancement by polarization transfer (DEPT) is a technique that provides information on the number of hydrogens attached to each carbon in a molecule. While the DEPT experiment utilizes complex pulse sequences, the pulse delay and flip angle are specifically manipulated. The resulting signals have different phases depending on...
1.8K
NMR Spectroscopy of Aromatic Compounds01:14

NMR Spectroscopy of Aromatic Compounds

6.7K
Aromatic compounds can be identified or analyzed using proton NMR and carbon‐13 NMR. Typically, aromatic hydrogens or hydrogens directly bonded to the aromatic rings are strongly deshielded by the aromatic ring current. Therefore, they absorb in the range of 6.5–8.0 ppm in proton NMR spectra. For instance, aromatic hydrogens directly bonded to the benzene ring absorb at 7.3 ppm. However, aromatic hydrogens of larger rings absorb farther upfield or downfield than the ideal range.
6.7K
¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

1.4K
At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
1.4K
¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

2.2K
A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied...
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Atomic Scale Structural Studies of Macromolecular Assemblies by Solid-state Nuclear Magnetic Resonance Spectroscopy
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Enhancing NMR Prediction for Organic Compounds Using Molecular Dynamics.

Eugene E Kwan1, Richard Y Liu1

  • 1Department of Chemistry & Chemical Biology, Harvard University , Cambridge, Massachusetts 02138, United States.

Journal of Chemical Theory and Computation
|November 18, 2015
PubMed
Summary

Quasical molecular dynamics accurately predicts NMR shieldings by accounting for thermal motion, improving chemical shift predictions for molecules without empirical corrections. This method enhances the study of dynamic molecules like [18]annulene.

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Area of Science:

  • Computational chemistry
  • Organic chemistry
  • Spectroscopy

Background:

  • Nuclear Magnetic Resonance (NMR) spectroscopy is vital for characterizing molecules and reaction mechanisms.
  • Conventional NMR shielding predictions often neglect the significant impact of molecular thermal motions.
  • Existing methods rely on stationary structures, limiting accuracy for dynamic systems.

Purpose of the Study:

  • To develop and validate a highly accurate method for predicting NMR shielding constants.
  • To investigate the influence of molecular dynamics on NMR spectral properties.
  • To extend the predictive power of computational methods to complex and dynamic molecules.

Main Methods:

  • Utilizing quasiclassical molecular dynamics simulations to incorporate thermal effects.
  • Calculating gas-phase absolute shieldings for protons and carbons.
  • Applying the method to small molecules, natural products, and the dynamic [18]annulene.

Main Results:

  • Predicted NMR shieldings closely match experimental uncertainties for small molecules.
  • Chemical shifts for large systems, including natural products, are accurately reproduced.
  • The method successfully predicts properties of [18]annulene across temperatures, confirming its planar, aromatic structure.

Conclusions:

  • Quasical molecular dynamics offers a broadly applicable and accurate approach to NMR shielding prediction.
  • The findings suggest that empirical corrections in current methods primarily account for vibrational effects.
  • This advancement enables reliable NMR analysis of molecules exhibiting significant dynamic behavior.